Which MHC complex is expressed only on professional antigen-presenting cells, and what does it bind?

Which MHC complex is expressed only on professional antigen-presenting cells, and what does it bind?

• A. MHC-II expressed on professional APCs; binds TCR and CD4
• B. MHC-I expressed on professional APCs; binds TCR and CD8
• C. MHC-II expressed on all nucleated cells; binds TCR and CD4
• D. MHC-I expressed on thymic epithelial cells; binds CD8

Answer: (A)

MHC class II molecules are restricted to professional antigen-presenting cells (dendritic cells, macrophages, and B cells) and are specialized to present exogenous peptides to CD4+ helper T cells. The T cell receptor recognizes the peptide–MHC II complex, with the CD4 co-receptor stabilizing the interaction and guiding activation of helper T cells that coordinate the immune response (including B cell help and macrophage activation). In contrast, MHC class I is found on nearly all nucleated cells and presents endogenous peptides to CD8+ cytotoxic T cells, so it isn’t limited to professional APCs. Therefore, the correct statement is that MHC II is expressed on professional APCs and binds TCR and CD4.

MHC class II molecules are restricted to professional antigen-presenting cells (dendritic cells, macrophages, and B cells) and are specialized to present exogenous peptides to CD4+ helper T cells. The T cell receptor recognizes the peptide–MHC II complex, with the CD4 co-receptor stabilizing the interaction and guiding activation of helper T cells that coordinate the immune response (including B cell help and macrophage activation). In contrast, MHC class I is found on nearly all nucleated cells and presents endogenous peptides to CD8+ cytotoxic T cells, so it isn’t limited to professional APCs. Therefore, the correct statement is that MHC II is expressed on professional APCs and binds TCR and CD4.